In response to PA-10-013: Behavioral & Integrative Treatment Development Program (R34), we propose to evaluate an mHealth-enhanced peer navigation intervention for improving retention in care and adherence to antiretroviral therapy (ART) for out-of-care, HIV-infected injection drug users (IDUs). This proposal is a Stage 2a study in the Stage Model of Behavioral Therapy Development, designed to collect feasibility and acceptability data for a multi-component intervention based on 2 promising strategies: peer navigation (PN) and ecologic momentary intervention (EMI). Despite the wide availability of ART, many HIV-infected IDUs are not engaged in HIV care. Personalized psychosocial and logistic support such as is commonly delivered by case managers or patient navigators can improve medical care for persons living with HIV/AIDS (PLWHA), but novel strategies are needed to ensure this support is financially sustainable over the long-term and responsive to the unique challenges encountered by substance abusing patients. We have developed the electronic Mobile Open-source Comprehensive Health Application (eMOCHA) to assist peer navigators (former IDUs successfully engaged in HIV care) in delivering individually-tailored interventions to HIV-infected IDUs who have been out of care for over a year. eMOCHA is an innovative, smartphone-based mHealth application that uses ecologic momentary analysis (EMA) methods to assess HIV care engagement (clinic attendance and adherence), drug cravings, and psychosocial stress. In this study, IDUs who attend an initial HIV care visit will be randomized to usual care or the eMOCHA-peer navigator intervention. According to predetermined algorithms, the eMOCHA program will detect threats to care engagement and trigger one of two types of interventions: (1) a phone call or visit from a peer navigator for assistance with a specific problem, or (2) an automated, context-specific text message (i.e., MI). Results from this study will allow refinement of the intervention and generation of preliminary efficacy data that will inform subsequent design of a larger, more expensive clinical trial to evaluate the effectiveness of this approach. If effective, strategies combining mHealth and peer navigation may offer a scalable, potentially cost-effective approach to keeping high-risk PLWHA engaged in care.